1. Field of the Invention
The present invention relates to an image display apparatus, such as a head mount display (“HMD”), which guides a plurality of luminous fluxes from a plurality of original images to different field areas in a single observation field, and displays a synthesis image.
2. Description of the Related Art
A small observation optical system is required for an image display apparatus, such as the HMD. In particular, a thin observation optical system is required in an observer's visual-axis direction. In addition, in order to enhance the realism of the image observations, the observation optical system is required to present an image having a wide angle of view.
The human field area is classified into a discrimination visual field (which is a region in which a sight function, such as an eyesight, is dominant or a region within several degrees from the center of an image), an effective filed (which is a region in which information can be searched only with eye movements or a region within horizontally ±15° and perpendicularly +8° to −12°), and an guide visual field (which is a region that affects a spatial coordinate system but provides only an identification capability that can identify an existence of the sense of sight and rough information of its identity or a region within horizontally ±50° and perpendicularly +350 to −50°).
According to O plus E, pp. 123-124, Physiological optics 16, January 1986, an angle of view suitable for the observation optical system is said to be an angle of view necessary to feel the realism of a wide screen image in the guide visual field. Hence, the field angle in the HMD can be a wide angle of view that covers a whole region of the guide visual field.
Nevertheless, it is difficult to constitute a small and lightweight observation optical system that covers the whole region of the guide visual field, and it is realistic to secure a field that is as close as possible to the angle of view of the guide visual field beyond the effective visual field.
Japanese Patent Laid-Open No. 07-333551 discloses an optical system suitable for a thin observation optical system. The optical system disclosed in Japanese Patent Laid-Open No. 07-333551 introduces a luminous flux from an optical image to a prism, folds an optical path on a plurality of decentering reflective and curved surfaces of the prism, and guides a luminous flux exited from the prism to a pupil. This optical system corrects a decentering aberration so that each surface can have a different optical power in accordance with an azimuth angle.
The optical system disclosed in Japanese Patent Laid-Open No. 07-333551 is small but introduces the luminous flux from the original image to the pupil without forming an intermediate image, and thus an achievable limit of the angle of view is small relative to a size of an original image displayed on an image forming device, such as a liquid crystal panel. Thus, use of a large image forming device having a wide original-image forming area is necessary to achieve a wide view angle.
When the image forming device is large, it is easy for an observation optical system to present an image having a wide angle of view. However, it is difficult to always obtain an image forming device having a proper size of an original-image forming area for a specification of a presented angle of view.
Japanese Patent Laid-Open Nos. 07-274097, 11-326820, and 10-246865 each disclose an observation optical system that can present an image with a wide angle of view while using an image forming device having an original-image forming area that is small relative to the angle-of-view specification. Each of these observation optical systems synthesizes a plurality of images formed by a plurality of luminous fluxes from the original image corresponding to different field areas, and presents the synthesized image in one observation field.
The observation optical system disclosed in Japanese Patent Laid-Open No. 07-274097 provides a V-shaped mirror to the interior of the optical system, reflects a luminous flux from one original image formed by one image forming device, on one mirror surface of the V-shaped mirror, and directs the luminous flux to the pupil. Japanese Patent Laid-Open No. 07-274097 also discloses an observation optical system that reflects the luminous flux from one original image, on one half-mirror in the V-shaped half-mirror, then reflects the luminous flux on the concave mirror, transmits the luminous flux thorough the one half-mirror in the V-shaped half-mirror, and directs the luminous flux to the pupil. This observation optical system folds the optical path of the luminous flux from the original image by using the other half-mirror surface in the V-shaped half-mirror and the concave mirror.
In Japanese Patent Laid-Open No. 07-274097, where a central angle-of-view principal ray is defined as a ray on an axis that connects the center of the pupil of the observation optical system to the center of the observation field on which the synthesis image is presented, the central angle-of-view principal ray passes an apex portion of the V-shaped half-mirror. Moreover, a section of an optical path of the central angle-of-view principal ray folded by the V-shaped half-mirror and the concave mirror is common to two original images.
In addition, the observation optical system disclosed in Japanese Patent Laid-Open No. 11-326820 uses a prism that integrates two prisms with each other, each of which is configured to introduce the luminous flux from one original image to the pupil, and the observation optical system presents two images formed by the luminous fluxes from the two original images in one observation field. These two optical systems, which correspond to two original images, and constitute the observation optical system, are configured so that sections that contain (folding) optical paths of the central angle-of-view principal ray folded by a plurality of decentering reflective and curved surfaces can accord with each other. In addition, these two optical systems corresponding to the two original images are configured to be arranged symmetrical with respect to a section that contains the central angle-of-view principal ray and is perpendicular to the sections that contain the folding optical path of the central angle-of-view principal ray.
Moreover, the optical paths from the two original images overlap each other in the observation optical system disclosed in Japanese Patent Laid-Open No. 10-246865, and an incident surface of the prism that opposes to the two image forming devices serves as a reflective surface that reflects the luminous flex from the other image forming device and directs it to the pupil. In other words, the optical surface included in each of the two optical systems corresponding to the two original images is commonly used for the luminous flexes from the two original images. The two optical systems are arranged symmetrical with respect to a section that contains the central angle-of-view principal ray, and is perpendicular to the section that contains the folding optical path of the central angle-of-view principal ray.
Thus, each of Japanese Patent Laid-Open Nos. 07-274097, 11-326820, and 10-246865 synthesizes, within one observation field, images formed by the luminous fluxes from the two original images by using the two optical systems that are arranged adjacently or in a partially overlapping state.
In order to reduce the size of the whole observation optical system (or a thickness in the visual-axis direction), it is effective that one optical system bears a small angle of view. On the other hand, when the angle of view which one optical system bears is small, an angle of view of a whole observation optical system cannot be made sufficiently wide.
The observation optical systems disclosed in Japanese Patent Laid-Open Nos. 07-274097, 11-326820, and 10-246865 use only two image forming devices and two optical systems, and thus one optical system bears a large angle of view. Thus, the thickness of the observation optical system becomes large, and it is difficult to make small and thin the image display apparatus.
In addition, the observation optical systems disclosed in Japanese Patent Laid-Open Nos. 07-274097, 11-326820, and 10-246865 arrange two optical systems adjacently, and thus the luminous flux from one image forming device is highly likely to reach the other image forming device. If the luminous flux from the one image forming device is reflected on the surface of the other image forming device, the reflected light may cause a ghost and/or a flare.
Moreover, the observation optical systems disclosed in Japanese Patent Laid-Open Nos. 07-274097, 11-326820, and 10-246865 use two image forming devices and two optical systems to make wide an angle of view in the horizontal direction.
The guide visual field has a higher ratio of a horizontal field to a perpendicular field than the effective visual field and the overall field sensible to the human. In the effective visual field, the horizontal field is wider than the perpendicular field. Hence, it is reasonable to maintain a wide field particularly in the horizontal direction. However, it is effective to realize a wide angle of view to some extent even in the perpendicular direction so as to improve the realism through a display that covers the whole guide visual field as thorough as possible.
In addition, since the observation optical systems disclosed in Japanese Patent Laid-Open Nos. 07-274097, 11-326820, and 10-246865 halves the field with the two optical systems, a seam exists at the field center between the two optical systems. Therefore, at the field center which an observer is highly likely to address, the quality of the presented image may degrade due to the discontinuity of the images, low brightness, flares that are generated from the unnecessary light that occurs at the seam.